This invention relates to an Ultrasound System and Method for Image Subtraction. In particular, a system and method for image subtraction during contrast agent imaging is disclosed.
Ultrasound systems image anatomy in various modes, such as B-mode imaging. Various aspects of imaging may be enhanced through the injection of contrast agents. The contrast agents may provide more intense echo signals at either fundamental or harmonic frequencies. Since the contrast agents flow through the various arteries, vessels and capillaries, B-mode imaging may be used to identify blood flow and perfusion. However, as acoustic energy from an ultrasound wave interacts with contrast agents, the contrast agents are destroyed or are altered to provide echo signals with a lesser intensity from contrast agents.
One method takes advantage of the destruction of contrast agents. Two successive firings are transmitted along a scan line before any transmission along another scan line. The first transmission results in the destruction of contrast agent and the second transmission results in echoes with little or no intensity provided from contrast agents. The data associated with the second transmission is subtracted from the data associated with the first transmission. This methodology is then repeated for a plurality of scan lines on a scan line by scan line basis to generate an image. See for example U.S. Pat. No. 5,456,257 to Johnson et al. and European Patent Application EP 0 770352. In one embodiment, a difference in frequency characteristics instead of B-mode intensity is obtained. However, line by line subtraction leads to increased collateral destruction of contrast agents along adjacent lines. As the transmissions are swept through the image plane, the multiple transmissions for each scan line multiply the collateral damage or destruction of contrast agent along an adjacent scan line. The intensity values associated with the first transmission along the second or adjacent scan line may underestimate the concentration of contrast agent.
In another method of contrast agent imaging, a frame of intensity data associated with a plurality of scan lines is obtained prior to the injection of contrast agent. The contrast agent is then injected. A second frame of data associated with the plurality of scan lines is obtained. The second frame of data is subtracted from the first frame of data to generate a frame of image data. See for example U.S. Pat. No. 5,255,683 to Monaghan. However, the delay between obtaining the first frame of data and the second frame of data after the contrast agent perfuses or otherwise flows into the target region may cause degradation due to changes in transducer position, motion in the anatomy and motion associated with the patient positioning or breathing.